1. Technical Field
An orthogonal process for patterning organic structures is disclosed which utilizes a fluorinated solvent or supercritical CO2 as a solvent for organic materials. The disclosed process may also utilize a fluorinated photoresist in combination with a fluorinated solvent. The fluorinated photoresist may be a resorcinarene, a copolymer of perfluorodecyl methacrylate and 2-nitrobenzyl methacrylate, derivatives thereof or other polymer photoresist or molecular glass photoresists having sufficient fluorine content. The fluorinated photoresist and fluorinated solvent are used to make patterns of various organic structures used in electronic (semiconductor) and electrical devices. For example, the materials and techniques disclosed herein may be applied to the patterning of acidic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a widely used organic material for which no other straightforward lithographic patterning method exists.
2. Description of the Related Art
The use of organic materials is becoming widespread in electronic and electrical device fabrication because organic materials can complement conventional inorganic materials to provide lightweight, inexpensive, and mechanically flexible devices. Advantages of using organic materials lie in the low-temperature, high-throughput fabrication processes for such organic materials. The fabrication of a variety of organic electronic and electrical devices such as organic light emitting diodes (OLEDs), organic thin film transistors (OTFTs), organic solar cells, electrodes and sensors has been demonstrated using spin coating, ink-jet printing, and other wet printing techniques.
Like traditional inorganic devices, devices made from organic materials require active functional materials to be tailored into micro-patterned and multi-layered device components. Traditional inorganic devices are typically made using known photolithographic patterning techniques, which provide high-resolution and high-throughput. However, organic semiconductors and other organic electronics or organic electrical structures cannot be made using known photolithographic patterning because of the chemical incompatibility between the organic materials and certain patterning agents, specifically, the solvents used during the patterning process. The use of conventional organic solvents deteriorates the performance of the organic materials during the photoresist deposition and removal. Further, the performance of organic materials deteriorates during the pattern development steps using conventional aqueous base solutions.
To overcome these problems, various strategies have been employed. One strategy is to modify the lithographic conditions to accommodate organic materials. These efforts include the employment of protective coatings between the active material and photoresist films. Other strategies include attempts to find an “orthogonal solvent,” or a processing solvent that does not deteriorate the organic layers. Alternative fabrication methods have also been employed including ink-jet printing, shadow mask deposition, vapor deposition through shadow masks, soft and hard imprint lithography, and photolithography. While ink jet printing boasts continuous roll-to-roll process capabilities and is the patterning technique of choice for polymeric materials, ink-jet printing resolution is limited to approximately 10-20 μm. Shadow mask deposition is the dominant technique for small molecule patterning, but also has notable resolution limitations, typically 25-30 μm at best, although special masks have shown resolution down to 5 μm. Shadow mask deposition also requires a high vacuum environment, which can introduce further limitations. Imprint lithography has demonstrated promising results, showing feature resolution down to 10 nm. However, this technique has only shown limited applicability with respect to materials and device architectures. Furthermore, all of the aforementioned methods suffer from lack of registration, which makes fabrication of multi-layer devices exceptionally challenging. Multi-layer device architecture is essential for achieving integrated circuits.
To date, no methods for the patterning of organic materials has been able to provide the resolution and dependability of photolithography. As a result, photolithography is the most widely-applicable patterning method that consistently achieves both high-resolution and registration. Photolithography has the added advantage of being the most developed patterning technology and the patterning method of choice of the semiconductor industry.
However, as noted above, there are a limited number of available solvents that do not dissolve or adversely damage an organic layer during a photolithography process. Currently, polar and non-polar solvents are used to process non-polar and polar active films respectively. For example, one can form a bi-layer structure by using a polar solvent to deposit a polar film on top of a non-polar film. Accordingly, solvent orthogonality can be achieved either by carefully choosing proper organic material/solvent combinations or by chemical modification of organic materials to achieve the desired polarity. This strategy is however problematic because both polar and non-polar solvents are typically required for photolithographic processes.
Hence, there is a need for an improved approach to the chemical processing of organic materials during the photolithography processes of an organic electronic device fabrication. Further, there is a need for environmentally friendly solvents that are benign to the majority of organic materials used in organic electronic device fabrication. Finally, there is a need for a robust method for processing organic electronic devices that avoids damage to the organic material such as dissolution, cracking, delamination or other unfavorable physical or chemical damage.